An Eye in the Sky Spies a Polluted Planet

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Looking down on Earth, instruments aboard a NASA satellite have
mapped out in unprecedented detail the pollution coming from the
world's megacities, metropolitan areas with more than 10 million
people, scientists report today (Sept. 22).

Using satellite data and wind patterns, the researchers were able
to gauge the nitrogen oxide pollution coming from megacities such
as Riyadh, the capital of Saudi Arabia. Though most of the
measurements matched previous estimates, study researcher Steffen
Beirle of the Max Planck Institute for Chemistry in Germany said
Riyadh emitted three times more pollution than expected.

"The key thing is that we applied a method in order to estimate
the lifetime and the [amount of] emissions without any
assumptions," Beirle told LiveScience. "Previous studies that
used satellite data had to involve models, and our method is
independent." [ Satellites
Gallery: Science from Above ]

Beirle and his colleagues used data from the Ozone Monitoring
Instrument (OMI) on the Aura satellite. OMI measures different
types of particles in the atmosphere, from dust to smoke to
sulfates. In this case, the researchers were interested in
nitrogen oxides, which come from vehicle emissions. Nitrogen
oxides contribute to acid rain and smog and are a big contributor
to urban air
pollution and health problems, Beirle said.

Earlier estimates of nitrogen oxide emissions relied on estimates
of the amount of fuel consumption and vehicular traffic in an
area rather than direct measurement of emissions. Using the
satellite to see what is there and wind pattern data to
understand how the pollution is moving, Beirle and his colleagues
are able to pinpoint the emissions.

The resulting visual representation of the data appears crude up
close, but the results are the most scientifically accurate yet,
the researchers say.

It's not yet known why some cities' measurements came in very
close to earlier estimates while Riyadh was worse than expected,
the researchers wrote in the Sept. 23 issue of the journal
Science.

So far, the method works only on cities of more than 10 million
in a relatively contained area. Moscow, Madrid and Tokyo all
worked well, Beirle said, but the method can't currently
calculate the nitrogen oxide from cities like New York or Hong
Kong.

"You need a certain amount of pollution so that you can clearly
see it from space, and it's also necessary that there are not
many interfering sources around," Beirle said. Riyadh, sitting
alone in the desert, makes a good candidate for monitoring. Hong
Kong or New York, close as they are to other urban pollution
sources, are still too complex for exact measurements.

The next step, Beirle said, is to refine the method so that
pollution from these complex urban areas can be measured. Having
good estimates
of pollution is important for modeling the chemistry of the
atmosphere, he said.

"These are our tools to understand what goes in the atmosphere,"
Beirle said. "They are also the guidelines for pollution-control
measures."